These are, or used to be, quite common with gcc. Unused-result warnings require dataflow analysis to run to determine whether the values are used, and on no-opt

These are, or used to be, quite common with gcc. Unused-result warnings require dataflow analysis to run to determine whether the values are used, and on no-optimisation compiles gcc didn't bother to do dataflow analysis. Conversely, here's one where a warning is produced only without -O2: orth$ cat z.c int foo (int a) { int x; if (a == 5) { x = 3; return 42; } return x; } orth$ gcc -O2 -Wall -o z.o -c z.c orth$ gcc -Wall -o z.o -c z.c z.c: In function ‘foo’: z.c:4:4: warning: ‘x’ may be used uninitialized in this function [-Wmaybe-uninitialized] return x; ^ orth$ gcc --version gcc (Debian 4.9.2-10) 4.9.2